Electronic, information, and communication industries have shown rapid development by manufacturing portable, small, light, and high-performance electronic devices, and demands for a lithium secondary battery that may exhibit high capacity and high performance as a power source of electronic devices have increased. A lithium secondary battery that is used while being charged/discharged by intercalation and deintercalation of lithium ions has been settled as an essential power source of medium-to-large sized devices such as electric vehicles as well as portable electronic devices for information and communication.
Graphite-based negative electrode active materials have been used as typical negative electrode materials of a lithium secondary battery, but a theoretical capacity of the negative electrode active materials is limited to 372 mAh/g. Thus, studies on high capacity materials such as silicon, tin, or a metallic complex thereof have been conducted. Also, interest in titanium and zinc oxide for considering high capacity and chemical safety, as well as a change in a nanostructure thereof having a high specific surface area has increased.
In particular, silicon is one of the high capacity negative electrode active materials of a lithium secondary battery, and a theoretical capacity of silicon based on reaction with lithium is about 4200 mAh/g.
However, silicon changes a crystalline structure during the reaction with lithium, and when silicon absorbs and stores the maximum amount of lithium during charging, the silicon converts into Li4.4Si, and a volume of silicon expands about 4.12 folds a volume of silicon before the expansion. Mechanical stress applied to silicon during the expansion generates cracks inside and on the surface of an electrode, and the silicon shrinks back when lithium ions are discharged by discharging. When the charging/discharging cycle repeats, pulverization of the negative electrode active material may occur, and the pulverized negative electrode active material agglomerates and electrically detaches from a current collector. Also, due to an increase in resistance caused by a large change of a contact interface among negative electrode active materials, a capacity of the battery rapidly decreases as the number of charging/discharging cycles increases, and thus the cycle lifespan of the battery may be shortened.
In order to resolve the problems above, methods of controlling rapid volume change in a silicon metal material by reducing a size of particles have been tried, and as one of the methods, a method including mechanically, finely grinding silicon and dispersing the resultant in a conducting material to prepare a Si—C complex as a negative electrode active material has been tried. Particularly, during nanoparticulation of silicon, surfaces of the particles easily oxidized, which formed an oxide coating layer on the silicon particles. Thus, the initial charging/discharging efficiency of a battery decreased, and a battery capacity was reduced as well. The problem caused by the oxidation was that, when a size of the particles decrease to a nanoscale, a fraction of an oxidation coating layer volume with respect to a metal volume increases.
In order to suppress the formation of an oxide coating layer and to improve conductivity, Patent Document 1 (Japanese Patent 2000-215887) discloses a method of coating a surface of silicon particles with a carbon layer by chemical vapor deposition. However, degradation in a current collecting property and cycle characteristic deterioration along with reduction in a large volume change accompanied by charging/discharging, as problems the silicon negative electrode needs to resolve, could not be prevented.
In addition, Patent Document 2 (Japanese Patent 2005-190902) discloses a method of reducing volume expansion by designing a stack structure of silicon active materials, and Patent Document 3 (Japanese Patent 2006-216374) discloses a method of compensating for the volume change by having an empty space between a core part formed of silicon particles and a porous outer part. However, the methods that increase cycle characteristics of a negative electrode material by coating a silicon surface include a process that is not economically efficient, and capacities thereof can only achieve those way lower than a theoretical capacity of silicon, which results in low battery performance.
In this regard, it is required to develop a negative electrode active material for a lithium secondary battery that may decrease pulverization of silicon particles, has a simple formation process, and is suitable for the use in portable phones or electric vehicles, in which repeated cycle characteristics are important, by decreasing a volume change of silicon particles accompanied by charging/discharging of the lithium secondary battery.